For this problem, I believe you can use the Pythagorean Theorem
The Pythagorean Theorem is a^2 + b^2 = c^2
Because you are looking for the hypotenuse, you should be able to plug the numbers in that way.
9^2 + 15^2 = c^2
81 + 225 = c^2
306 = c^2
Find the square root for 306 (Its going to be a multi-decimal number)
x = 17.5 (when rounded to nearest tenth)
x = 17 (when rounded to the nearest whole number)